With this method, the well is killed in two circulations. During the first circulation: the influx is circulated out of the hole using the existing mud. Additional influx is prevented by adjusting the choke to maintain a constant bottom hole pressure slightly in excess of the pore pressure.

During the second circulation: the existing mud is replaced by mud of the required density to (over)balance the pore pressure. The choke is adjusted to maintain a constant bottom hole pressure slightly greater than the pore pressure.

1. Disadvantages of the driller's method

Compared with the balanced mud method, principal disadvantages of the driller's method include the following:

·the well must remain closed-in under pressure longer;

·the maximum pressure at the casing shoe and against the formation will be higher if the influx is gas (unless the top of the gas reaches the casing shoe before the drillstring would be displaced by heavy mud in the balanced mud method);

·the maximum choke pressure when the top of the influx reaches the surface will be higher if the influx is gas.

Before employing the driller's method, it is essential to confirm that exposed formations can support the higher pressures which might be developed during the first circulation.

2 Advantages of the driller's method

Advantages of the driller's method include the following:

·simplicity: circulation can be started without calculations. This may be useful if expert supervision is not immediately available;

·pumping can begin as soon as drillpipe pressure build-up is established; there is no delay whilst mud is weighted up. This could be important in case of an H2S influx;

·the well can be effectively controlled (although not killed), even if the weighting material supply is inadequate.

2.1 Procedures

The following procedures concerning the driller's method are discussed:

·Closing in the well.

·Pressure and pit volume readings.

·First circulation: selecting the pump rate.

·Standpipe pressure during first circulation.

·Determining the height and gradient of the influx.

·First circulation: determining travel times (or volumes).

·First circulation: standpipe kill graph construction and use.

·Determining the pressure at the top of a gas influx at any point in the annulus.

·First circulation: action.

·Second circulation: determining the gradient of the kill mud.

·Second circulation: determining the amount of overbalance.

·Second circulation: selecting pump rate.

·Second circulation: travel times (or volumes).

·Second circulation: standpipe pressures.

·Second circulation: standpipe kill graph construction and use.

·Second circulation: action.

·Procedure after the well has been brought under control.

3 Closing in the well

Close in the well immediately after detecting a kick condition. The procedure is as for the balanced mud method.

4 Pressure and pit volume readings

Pressure and pit volume readings should be taken as for the balanced mud method

5 First circulation: Selecting the pump rate

The mud is not weighted up for the first circulation: therefore, the pump rate is not limited by the weighting material mixing capacity of the rig. However, the maximum pump rate is limited by other factors such as the increased initial standpipe pressure, the need for choke adjustment, and surface gas handling equipment. Also, if the choke starts blocking-off, pressure surges will be less at reduced circulating rates. Normally, the pump speed selected will not exceed 50% of the usual circulating rate applied for drilling operations.

6 Standpipe pressure during first circulation

The standpipe pressure at the start is the same as with the balanced mud method. The standpipe pressure should then be approximately equal to the normal pre-kick circulation pressure at the selected pump speed, plus the closed-in drillpipe pressure, plus a small margin of 700 kPa (100 psi).

Always make sure that the formation strength at the casing shoe is not exceeded during the circulating process.

Since there is no change in the gradient of the mud being pumped, the initial standpipe pressure must be held constant throughout the first circulation to ensure that the bottom hole pressure is also kept constant.

7 Determining the height and gradient of the influx

This information is not essential, but will give an indication of the pattern of choke pressures and pit level changes that may be expected during the first circulation. The procedure is as for the balanced mud method.

8 First circulation: Determining travel times (or volumes)

The bit-to-shoe and shoe-to-choke times are determined .The total pumping time for the first circulation is that required to displace the annulus, i.e. the sum of the bit-to-shoe and shoe-to-choke times, volumes, or pump strokes.

9 First circulation: Standpipe kill graph construction and use

The standpipe kill graph is a horizontal line equal to the closed-in drillpipe pressure plus the circulating pressure plus the overbalance margin of 100 psi.

10 Determining the pressure at the top of a gas influx at any point in the annulus

When a gas kick is being circulated out of the hole, the influx volume will increase due to expansion and consequently results in increased pit levels.

By calculating the expected annular pressures at the top of the influx at specific points along the hole together with the associated influx volumes at these points, comparisons can be made with actual values observed during circulating out the influx. This information can play an important role in the decision making process during well control operations.

The pressure at the top of a gas bubble at any point in the annulus while circulating it out using the "Driller's method" can be calculated as follows:

11 First circulation: Action

The procedure for the first circulation is as follows:

1.Open the choke and start pumping the existing mud at the selected pump speed.

2.Adjust the choke opening until the choke pressure equals the closed-in annulus pressure plus the overbalance margin. Record the choke pressures throughout the first circulation.

3.Read the standpipe pressure. It should agree with the calculated value, i.e. the normal pre-kick pump test circulation pressure at the selected pump speed plus the closed-in drillpipe pressure, plus a small margin of 700 kPa (100 psi). If the observed standpipe pressure does not agree with the calculated value, consider the observed pressure to be correct.

4.Note the standpipe pressure and thereafter keep it constant whilst maintaining a constant pump rate, until the influx is circulated out.

5.When all influx has been circulated out, stop the pump and close in the well to check the closed-in drillpipe and annulus pressures. At the end of the first circulation, the closed-in pressures of the annulus and drillpipe should be the same and equal to the initial closed-in drillpipe pressure. The well is controlled but not killed.

During the first circulation the following should also be carried out:

·maintain and record the density of the mud pumped into the drillstring. Ensure that it has the correct value;

·measure and record the properties of the mud returns;

·de-gas, treat or discard any contaminated mud returns.

12 Second circulation: Determining the gradient of the kill mud

The gradient of the kill mud to balance the formation pressure can be determined as soon as the closed-in standpipe pressure has stabilised. A trip margin can now be added to the kill mud gradient in order to overbalance the formation pressure and to resume normal operations.

13 Second circulation: Determining the amount of overbalance

Normally the overbalance on bottom during well control (neglecting friction losses in the annulus), should not exceed 700 kPa (100 psi). However, since the influx has been displaced with r1 mud during the first circulation, large fluctuations in mud gradient and choke control operations are not expected and therefore, if possible, the density of the mud in the well is raised directly to that required to resume normal operations.

14 Second circulation: Selecting pump rate

This is carried out as for the balanced mud method.

A constant pump rate, approximately one half the speed used for the drilling operation, is maintained during the second circulation.

15 Second circulation: Travel times (or volumes)

Provided that the same pump rate is used, the surface-to-bit and bit-to-choke times are the same as for the balanced mud method Section

16 Second circulation: Standpipe pressure

The initial standpipe pressure should be the same as for the first circulation.

Pst =Pdp + Pc1 + margin

During the period that the heavy mud (including the overbalance) is pumped down the drillstring, the standpipe pressure should decrease until the heavy mud reaches the bit at which time it should be:

Pst = Pc1 * rho2 / rho1 = Pc2

The standpipe pressure should remain constant after the heavy mud has reached the bit.

17 Second circulation: Standpipe kill graph construction and use

The standpipe pressure kill graph for the second circulation is similar to that of the balanced mud method

The procedure for constructing the standpipe kill graph is as follows:

1.Plot the initial circulating pressure plus margin at the start of the second circulation.

2.Plot the heavy mud circulating pressure (Pc2) at the time that the heavy mud reaches the bit.

3.Whilst the heavy mud is being circulated into the annulus, the back pressure should be progressively reduced to zero at the time when the heavy mud reaches the choke. The standpipe pressure should then equal the heavy mud circulating pressure.

This assumes that the heavy mud gradient includes a suitable overbalance margin.

18 Second circulation: Action

If possible, the density of the mud in the well is raised directly to that required to resume normal operations.

The procedure during the second circulation is as follows:

1.Open the choke and start pumping mud of the required density at the rate selected to kill the well. Maintain a constant pumping rate.

2.Adjust the choke opening until the choke pressure equals the closed-in annulus pressure plus margin observed at the end of the first circulation. Choke pressures should be recorded throughout the process.

3.Read the standpipe pressure. This should agree with the calculated standpipe pressure, i.e. the pre-kick pump test circulating pressure plus the closed-in drillpipe pressure at the end of the first circulation including the margin. If the standpipe pressure does not agree with the calculated value, consider the observed pressure to be correct and modify the standpipe pressure kill graph accordingly.

4.When the heavy mud reaches the surface, stop pumping and check whether the well is dead.

During the second circulation the following should also be carried out:

·maintain and record the density of the mud pumped into the drillstring; ensure that it has the correct value;

·measure and record the properties of the mud returns until the well is killed;